I don't know for which aircraft are you asking but on 737 if you arm both AP for approach, you get quite a bit of nose-up trim at ~ 400ft RA to help the aircraft with flare in autoland and could come as a nasty surprise as you disconnect the autopilot at Cat 1 minima in marginal weather for manual landing. I believe this is the reason why most operators choose to fly ILS approaches with only one autopilot unless conducting autoland.